This invention relates to polylactides in a purified state, to the method of their purification and to pharmaceutical compositions comprising the purified polylactides.
The European Patent Application No. 0270987 A2 describes polylactides, e.g. polylactide-co-glycolides, which have been prepared by condensation of lactic acid and glycolic acid or preferably by polymerisation of lactide and glycolide in the presence of a catalyst, e.g. tin di-(2-ethyl hexanoate), also known as tin octoate or tin octanoate.
The polylactides are purified by dissolving them in a solvent, which is not or is only partially miscible with water, e.g. methylene chloride, and washing the solution with an aqueous solution of an acid, e.g. HCl or of a metal ion complexing agent e.g. EDTA, after which the catalyst metal cation or its complex is transferred to the aqueous solution, in which it is better soluble.
However, after separation and isolation of the organic solvent layer and the precipitation of the polylactide from it e.g. by mixing the layer with an organic solvent, e.g. petrol ether or an alcohol, e.g. methanol, which dislodges the polylactide from the solution, the precipitated polylactide still contains a certain amount of the catalyst metal cationxe2x80x94about 2 ppmxe2x80x94and additionally the catalyst anion, e.g. in acid form. Further the polylactide contains a certain amount of brown coloured decomposition by-products which have been formed in the polymer preparation process, especially under the influence of the catalyst.
Since polylactides, e.g. polylactide-co-glycolides, are preferably used as matrices for drug compounds e.g. in implants or microparticles, which are administered parenterally, the remaining impurities can give rise to local irritation reactions of the body tissue and, e.g. depending on the catalyst type, to an instability of the matrix and thus possibly to an accelerated drug compound release. The brown impurities and the catalyst should thus preferably be removed as good as possible.
Catalyst-free polylactides on the basis of the condensation of lactid acid and optionally glycolic acid are known, but have low molecular weights of about 2000 to 4000.
Polylactides of a higher molecular weight are preferred and can only be made in the presence of a catalyst.
According to the European Patent No 0026599 B1 lactic acid and glycolic acid were reacted in the presence of a strong acid ion-exchange resin as a catalyst, which after the reaction could be removed from the copolymer product by filtering the molten reaction mixture or by cooling the reaction mixture, dissolving the copolymer in an organic solvent in which the ion-exchange resin is insoluble, filtering the solution and removing the organic solvent after which a copolymer was obtained from which the solid phase catalyst was removed to a substantially extent.
However, by this method only polylactides were obtained having a molecular weight of from about 6000 to 35000.
Polylactides, e.g. polylactide-co-glycolides, having a broader molecular weight range than up to 35000 are preferably made by using lactide and optionally e.g. glycolide as monomers, but are polymerised in the presence of a metal catalyst, which reaction type, as has been discussed before, leads to a considerable contamination of the reaction product.
We have found now, that polylactides, e.g. polylactide-co-glycolides, especially those prepared from lactide and glycolide as monomers, can be obtained in a better purified state. The invention provides a polylactide in a purified state which meets the requirements of
the colour strengths of reference solutions B2-B9 of the brown colour test of the European Pharmacopoeia, 2nd Edition (1980), part I, Section V, 6.2. and
containing one or more metals in cationic form, the metal ion(s) having a concentration of at most 10 ppm.
The polylactide preferably has the reduced colour strengths of the reference solutions B4-B9, especially of reference solution B9. The colour of a reference solution B9 indicates that the polylactide is an off-white or colourless product.
The polylactides which are preferably prepared contain particularly bivalent metal ions, like Zn++ and especially Sn++.
For the determination of the tin amount, the polymer is decomposed under high pressure with a mixture of hydrochloric acid and nitric acid. The precipitation and concentration of tin from that mixture occurs on a membrane filter and the measurement of the metal amount is carried out by energy dispersive X-ray fluorescence (EDXRF), as described by H. D. Seltner, H. R. Lindner and B. Schreiber, Intern. J. Environ. Anal. Chem., 1981, Vol. 10, pp. 7-12 supplemented with a reference graphite furnace atomic absorption spectrometry method, as discussed on the 6th Colloquim Atomspektrometrische Spurenanalytik, Apr. 8-12th, 1991 in Konstanz, Germany, Authors: H. Seltner, G. Hermann and C. Heppler.
According to the invention the concentration of Sn++ in the purified polyactide of the invention is preferably at most 1.5 ppm, particularly at most 1 ppm; the catalyst anion is preferably ethyl-hexanoate, which is in the purified polylactide of the invention preferably present in a concentration of at most 0.5% by weight of the polylactide.
The purified polylactide preferably contains apart from its lactide units further structural units e.g. such as described in the european Patent Application No 0270987, second passage on page 4, of which the glycolide unit is the preferred unit since, depending on its monomer ratio in the polymer chain, it can shorten the decomposition period of the polymer in the body and thus accelerate the drug compound release time. The glycolide unit is, as is known, the most frequently used additional unit in polylactides.
The monomer molar ratio""s of the lactide/glycolide units in the purified polymers according to the invention are preferably 100-25/0-75, particularly 75-25/25-75, especially 60-40/40-60, more especially 55-45/45-55, e.g. 55-50/45-50.
It is known that the polymerisation reaction of monomers like lactide and glycolide is preferably carried out in the presence of a compound having one or more hydroxylgroups, which functions as a starter in building up a linear polymer chain. Known starters are e.g. lactic acid and glycolid acid. Other hydroxyl group containing compounds can also be used, e.g. alcohols. The starters are in fact used to control the chain length of the polylactides. A smaller amount of starting hydroxyl compound leads to longer chains than greater amounts. Excellent regulators are polyols, e.g. those described in the UK Patent Application GB 2.145.422A, of which mannitol and especially glucose are the most preferred.
By using this type of starting compounds relatively high molecular weight hard polylactide-co-glycolide materials can be obtained, which are very suitable as implants or microparticle materials, and have 2 or 3, preferably more than 3, e.g. 4 relatively short polylactide-co-glycolide chains and can hydrolyse in the body tissues within a relatively short drug release period of some weeks to e.g. 2 months or more, preferably within 4-6, e.g. 5 weeks. Although according to the invention the purified polylactides can have a linear structure, the preferred purified polylactides according to the invention are those having the structure described in the GB Patent 2.145.422 A, being esters of a polyol containing at least 3 hydroxyl groups, preferably those being an ester of a sugar or a sugar alcohol, especially an ester of glucose. They are star shaped, having a centre of e.g. the glucose rest and rays of linear polylactide chains.
After their preparation the star polymers are, more than the linear polymers, contaminated by brown-coloured by-products, since the sugar or sugar alcohol, used for their preparation, is also be partially decomposed by the catalyst. The star polymers have monomer molar ratios of lactid/glycolide units which are preferably those, indicated above for the linear polymers.
The star polymers have preferably a mean molecule weight Mw of from 10000 to 200000, especially of from 25000 to 100000, particularly of from 35000 to 60000, e.g. about 50000 and preferably have a polydipersity Mw/Mn of from 1.7 to 3.0, especially from 2.0 to 2.5. Poly-lactide-co-glycolides of a linear structure, not being star polymers in a purified state according to the invention have preferably a mean molecular weight Mw of from 25000 to 100000 and have preferably a polydispersity Mw/Mn of from 1.2 to 2.
The molecular weight Mw is determined by gelpermeation chromatography, using polystyrene as a standard, e.g. Dupont Ultrastyragel R 1000 or 500 Angstrom, in the column and e.g. tetrahydrofuron as the solvent.
The purified polylactides according to the invention can be obtained in a new process by contacting a solution of the impure polylactide with active charcoal and isolating the purified polylactide from the eluate. This process is also a part of the invention.
It is known from the UK Patent 1.467.970 and the EP 0181.621 A2 to treat polymers, produced in the presence of a catalyst with active charcoal.
According to the GB Patent 1.647.970, the polymer is a polyether, obtained from alkylene oxides, like ethylene oxide, propylene oxide or epichlorohydrin with an active hydrogen containing compound, e.g. glycerol, sorbitol or sucrose, in the presence of a basic catalyst. The polymer is purified with a mixture of active charcoal and magnesium silicate to remove crystals of polyalkylene glycols, e.g. polyethylene glycols, formed as by-products and which give the polymers a cloudy appearance and unsatisfactory viscosity and chemical properties. In a preferred method the polyethers are pretreated by means of a not further described other purification method to remove unreacted alkylene oxides and catalyst (page 2, lines 16-20). The basic catalyst thus could clearly not be removed by the charcoalxe2x80x94magnesium silicate mixture.
According to the EP 0181 621 A2 a solution of a polyalkylene ether in a cyclic ether or in a polyhydric alcohol solvent or the polyalkylene ether itself, e.g. polyoxytetramethylene glycol, prepared by the polymerisation of tetrahydrofuran under the influence of a heteropoly acid catalyst, e.g. 12-tungstophosphoric acid, is mixed with an organic hydrocarbon or halogenated hydrocarbon solvent. This solvent, which will contain the greatest part of the heteropoly acid, is separated from the other phase and the residue is contacted for further purification with a solid adsorbent, like charcoal, aluminium oxide or oxides, hydroxides or carbonates of e.g. Mg or Ca or with basic ion-exchange resins. According to Table 2 on page 23, the polymers contain 0.2 to 1.8 ppm of acidic metal contamination after purification with active charcoal.
This purification process is thus used to purify a polyether and to remove a very specific acidic catalyst type and it could not be foreseen, that active charcoal can be used for the removal of cations, like Sn++. Also it could not be foreseen, that such low impurity levels can be obtained as indicated.
The amounts of active charcoal used according to the purification process of the invention are generally from about 10 to 200%, e.g. 70 to 150% of the polymer weight. Any available charcoal can be used, e.g. as described in the Pharmacopoeia. A representative charcoal type is Norit of Clydesdate Co. Ltd., Glasgow/Scotland.
Typically powdered charcoal is used e.g. finely ground charcoal wherein at least 75% passes through a 75 micrometer sieve. Suitable charcoals as used in the Example hereafter, are described in brochures, available from Norit, e.g. xe2x80x9cSummary of methods for testing Norit activated carbons on specificationsxe2x80x9d by J. Visser.
The new purification process with charcoal is especially of interest for star polymers which have a dark brown colour. The colour effect may partly be caused by the polyol, e.g. the glucose, being instable to heat, especially in the presence of a catalyst and is more pronounced than a reference solution of colour strength B1.
It is believed that the presence of small amounts of acidic groups in the active charcoal are responsible for the surprisingly efficient removal of the cations. If a solution of the catalyst in an organic solvent is treated with charcoal, the catalyst compound is decomposed and the tin is removed with the charcoal, which is filtered of, whereas the anionic part of the catalyst is quantitatively found back in the remaining solution.
For this reason the invention also provides a method for the purification of the polylactide by contacting a solution of the impure polymer with a matrix having on its surface acidic groups and isolating the purified polylactide from the eluate.
If desired a weakly acidic cationic exchanger with a carboxylic acid functionality may be used if it is of an appropriately small particle size. For example, one with a hydrogen ionic form, a density when wet of about 0.69 g/ml (apparent) and 1.25 g/ml (true), a shipping weight of 690 g/liter, an effective particle size of 0.33 to 0.50 mm, a moisture content of 43 to 53 percent, a pH tolerated range of 5 to 14, a maximum operating temperature of 120xc2x0 C., a total exchange capacity of 10 meq (dry) and 3.5 meq/ml (wet). An example is Amberlite IRC-50 Methacrylic aci DVB (available from Fluka, Switzerland) which is ground to have the particle size diameter reduced e.g. to below 1 mm or 100 microns.
The matrix for the purification, e.g. the charcoal, preferably has a suitable concentration of from 0.01 o 0.1 millimole of acidic groups per gram of matrix and is conveniently in the form of particles which may be finely divided. Typical particle diameters are e.g. from 1 micrometer to 1 mm, e.g. from 10 to 100 micrometers.
They have therefore a large surface area. For example active charcoal has a surface area of 1000 square metres for each ml of matrix substance.
The purification process of the invention is preferably related to a polylactide preparation using lactide and glycolide as monomers and metal cations, like Sn++ as a catalyst, since this polymerisation process gives a better yield and, if desired, a higher molecular weight than the preparation process using lactid acid and glycolic acid as starting compounds and the strong acid ion-exchange resin as a catalyst, described in the European Patent No. 0026599.
Starting with an impure polylactide-co-glycolide containing about 1800 ppm of Sn++, the concentration of Sn++ can be lowered to about 200 ppm. A purification with charcoal can lower the Sn++ content, as already mentioned, to less than 1.5, e.g. less than 1 ppm.
The purification process of the invention is preferably carried out with a solution of an impure polylactide in acetone although other solvents are possible.
The process may be followed by another purification process, preferably the process of ultrafiltration, which reduces the content of low molecular compounds, e.g. of lactide and glycolide. Also in this process a polylactide solution in acetone can be used.
After the second purification process purified polylactides can be obtained, having a monomer content of at most 1% by weight of polymer, preferable of at most 0.25% of polymer, e.g. of at most 0.2 % of lactide and 0.05% of glycolide, a water content of at most 1%, an organic solvent, e.g. methylene chloride or acetone, content of at most 1%, preferably of at most 0.5% e.g. of at most 0.3% and an ash content of at most 0.1% by weight of polylactide. Their acid number is preferable at most 10. The thus purified polylactides are preferably parenterally used, e.g. as a matrix for drug compounds, especially such in implants or in microparticles form. These forms may be prepared in conventional manners, which have abundantly been described in the literature e.g. in the European Application No 58481, the UK Patent Application GB 2.145.422, the European Patent Application No 52510 the U.S. Pat. Nos. 4,652,441 and 4,711,782, the French Application No 2.491.351, the U.S. Pat. No. 3,773,919.
The forms are suitable for e.g. incorporating a hydrophilic drug like a peptide, e.g. a cyclopeptide and particularly a hormonally active peptide, like a somatostatin, especially octreotide, or an acid addition salt or a derivative thereof, or a lipophilic drug, like an ergot alkaloid, e.g. bromocriptine.
The pharmaceutical compositions are formed by working up the purified polylactide with the drug compound to form an implantate or a microparticle.